Transformation pathways from calcium sulfite to α-calcium sulfate hemihydrate in concentrated CaCl2 solutions

• α-HH evolves from CSH via two pathways (i.e. CSH → DH → α-HH and CSH → α-HH) in concentrated CaCl2 solutions.
• Competitive nucleation between DH and α-HH accounts for the phase evolution.
• The formation of DH was attributed to the faster heterogeneous nucleation.
• This paper provides a potential method for sulfite-rich FGD scrubber sludge utilization.

The disposal of sulfite-rich flue gas desulfurization (FGD) scrubber sludge has been a tough problem. The sludge now finds its prospective utilization way by being transformed into α-calcium sulfate hemihydrate (α-HH). At low pH, atmospheric pressure and 95.0 °C, the calcium sulfite hemihydrate (CSH) is oxidized and then changed to α-HH via calcium sulfate dihydrate (DH) in 2.50–3.50 m CaCl2 solutions (i.e. CSH → DH → α-HH). Direct transformation from CSH to α-HH (i.e. CSH → α-HH) occurs in a 4.00 m CaCl2 solution. The formation of DH at low CaCl2 concentrations is probably attributed to faster heterogeneous nucleation. The increment in CaCl2 molality lowers the water activity, and enlarges the supersaturation gap between DH and α-HH nucleation, which makes α-HH nucleation more competitive. Higher CaCl2 concentration favors the α-HH formation, and nucleation kinetics accounts for the transformation pathway evolution. The preparation of α-HH from the sulfite-rich FGD sludge is of great sense to electric power industry and environmental conservation.